1. Field of the Invention
This invention relates to a service valve and more particularly to a rotary plug-type service valve molded of a high strength plastic material and having a unitary plug and actuator member rotatable in a valve seat of the valve body where the actuator contacts the valve body over a substantial area so that external impact forces applied to the actuator are transmitted to the valve body and not localized on the actuator.
2. Description of the Prior Art
It is well known to fabricate a service valve, such as a ball valve or a plug valve, of molded polyethylene to include a one-piece body having a valve seat with a first passage opening to the seat and a second passage opening to the seat. A rotary member is positioned in the valve seat and a passageway through the rotary member is movable between a closed position where the flow between the first and second passages of the valve body is blocked and an open position where the passage through the rotary member is aligned with the passages in the valve body to permit flow between the first and second passages through the rotary member. The rotary member is provided with seat seals which seal against the wall surrounding the valve body passages to shut off the flow through the passages when the rotary member is turned to the closed position.
It is also known to provide an integral stem and plug arrangement where the stem extends upwardly from the plug and is connected to an adapter for turning the stem. The unitary stem and plug are molded of polyethylene. Because the stem projects upwardly and through the valve body to support the valve actuator, the stem is exposed to external impact blows which are transmitted downwardly through the pressure boundary in the valve to the plug. Accordingly, the stem must have a sufficient cross-sectional area to withstand the impact blows, as well as, withstand the torsional forces transmitted by the valve actuator during operation of the valve.
In view of the fact that polyethylene is a relatively soft and medium-yield material, a polyethylene valve stem must have a thickness substantially greater than the wall thickness of the polyethylene valve body.
It is the conventional practice to fabricate the known polyethylene plug and ball valves by an injection molding process. An integral stem and plug has specific structural requirements that substantially complicate the molding process. Additionally, because polyethylene is a relatively soft plastic material, the valve parts must include relatively thick sections to safely transmit operating forces. These thick valve sections cause the unit cost to increase because increased injection molding machine time is required to allow the thicker sections to cool once the mold is filled with injected plastic. The tooling cost associated with this specialized molding process is high. Consequently, the overall manufacturing time and cost of fabricating a polyethylene service valve mold are high.
An additional problem encountered in the operation of polyethylene service valves is the tendency of the valve body parts to creep or migrate away from one another when subjected to high pressure, particularly when the pressure is applied to the seat seal on one side of the plug without a corresponding supporting pressure on the outer side of the plug. One solution to controlling this migration or creep is reinforcement of the portion of the valve body that surrounds the plug seat seals. By reinforcing the valve body, the normal tendency of the elastomeric material to creep or separate is overcome by rigidifying the valve body.
In one method, the valve body is rigidified by providing an enlarged, integral, external reinforcement ring around the valve body opposite the valve seat. It is also known to provide internal reinforcement rings embedded in a valve body around the valve seat. This reinforced structure is designed to prevent separation or migration of the elastomeric material of the valve body away from the plug member around the valve seat.
The provision of molded, external, as well as, internal reinforcement rings on a valve body requires specialized molding of the valve body to accommodate the reinforcing structure at localized points externally or internally. The molded valve body, therefore, does not have a uniform thickness throughout. The mold for the valve body is considered to be complex because of the need to provide for variations in the wall thickness of the valve body. Such a mold requires complex machine operations to fabricate. Therefore, there is need in a plastic service valve for an integral stem and plug capable of withstanding impact loads directed upon the valve stem, as well as, torsional forces transmitted during operation of the valve and is economically fabricated to resist migration or creep of the plastic material.